Why does Facebook and YouTube not recognise that I have the latest Firefox?

When ever I am on my Mac desktop it doesn't recognize the latest version of Firefox. I have deleted and downloaded new version over and over. Websites like Facebook, Google Chrome and YouTube also tell me that I don't have the latest version so change their interface to reflect this. My laptop is fine it's only my desktop.

When ever I am on my Mac desktop it doesn't recognize the latest version of Firefox. I have deleted and downloaded new version over and over. Websites like Facebook, Google Chrome and YouTube also tell me that I don't have the latest version so change their interface to reflect this. My laptop is fine it's only my desktop.

You posted her with what looks like a real old version:
User Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.7; en-US; rv:1.9.0.19) Gecko/2010031218 '''Firefox/3.0.19''' FBSMTWB
FBSMTWB = Fast Browser Search was messing with the Firefox UA back in 2010 - 2011 and that hack has been carried thru in that Profile since then.
See this support article to learn how to fix it.
https://support.mozilla.org/en-US/kb/how-reset-default-user-agent-firefox

You can copy files like these to the current Firefox profile folder to recover specific data.

bookmarks and history: places.sqlite

bookmark backups: JSON backups in the bookmarkbackups folder

SQLite files like cookies.sqlite (cookies) and formhistory.sqlite (saved form data)

logins.json and key3.db (decryption key) for Passwords saved in the Password Manager

cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager

persdict.dat for words you added to the spelling checker dictionary

permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences

sessionstore.js for open tabs and pinned tabs and tab groups

Maybe it is a good idea to create a new profile if your current profile folder is that old.
That way you remove all older files and no longer used preferences stored in the prefs.js file.
See "Creating a profile":
*https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
*http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues
You can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.
*http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox
You can copy files like these to the current Firefox profile folder to recover specific data.
*bookmarks and history: places.sqlite
*bookmark backups: JSON backups in the bookmarkbackups folder
*SQLite files like cookies.sqlite (cookies) and formhistory.sqlite (saved form data)
*logins.json and key3.db (decryption key) for Passwords saved in the Password Manager
*cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager
*persdict.dat for words you added to the spelling checker dictionary
*permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences
*sessionstore.js for open tabs and pinned tabs and tab groups